After the retirement of the space shuttle fleet, the next generation of reusable launch vehicles (RLV) will require new and innovative materials for weight and cost savings. One of the major components of an RLV is the cryogenic tanks for propellant storage. The development of new lightweight tanks has been researched for several years, with lightweight carbon fiber composites being the major focus [1, 2]. However, during development of composite cryogenic tanks, there have been a couple of incidents involving dramatic tank failure during testing [3, 4]. Notice of impending failure would have prevented the cost in injuries and damage to facilities. Current structural monitoring technologies typically use metal foil strain gauges but which have significant limitations such as measuring strain in designated directions and locations, and are susceptible to drifting due to temperature sensitivity. Also, traditional strain gages have an extremely high failure rate and therefore are not well suited for long term use to determine the “health” of a composite structure. Furthermore, many structures such as airframes and cryogenic tanks do not allow access to place or change strain gages if failed.
Ever since their discovery, carbon nanotubes (CNTs) have been researched extensively due to their exceptional electrical and mechanical properties as potential candidates for many applications such as nano-sensors, nanoelectromechanical devices, switches, carbon nanotube-based oscillators and many more [5-9].
The use of CNT films for strain sensors for structural health monitoring has been proposed and tested [10-14] with excellent results, however in these cases, no specific applications were addressed. Development of an embedded strain sensor in a composite structure for use in future transportation vehicles will allow static and dynamic responses without compromising the host structure was reported by Park et al. [12]. One concern has been that CNT films are sensitive to temperature that may introduce errors in a widely varying temperature environment. Results from Vemura et al. [14] showed multi-wall CNT (MWCNT) films exhibited a decrease in resistivity with increasing temperature, but it was stable and predictable, varying by only 0.0217 ohm for a temperature change of 21.1° C. to 35° C. Similarly, Koratkar et al. [15] observed a small decrease in resistance with increasing temperature of a vertically aligned MWCNT film investigated in the temperature range −150° C. to 300° C.
Additionally, carbon nanotubes and especially multi-wall carbon nanotubes (MWCNTs) were incorporated into different types of polymers creating new nano-composite materials with enhanced mechanical and electrical properties [16, 17]. It is important to mention that the parameters such as nanotube crystallinity, length, concentration, interaction between the nanotubes, and the polymer matrix strongly affect the mechanical properties of these new CNT-reinforced polymer composites [18, 19]. Carbon nanotubes synthesized by chemical vapor deposition were found to be very tough and strong and have an extremely high Young's moduli (in the TPa range) by several groups around the world [20, 21]. Wong et al used an atomic force microscope to determine the mechanical properties of isolated MWCNTs [22]. Through a series of continued bending movements, the Young's modulus of the CNTs was measured to be about 1.28 TPa, independent of the nanotube diameter [22].
To characterize a series of individual MWCNTs, Yu et al performed a tensile-loading experiment within a scanning electron microscope, where the highest tensile strength and the Young's modulus of the outer most tube of a MWCNT were found to be 63 GPa and 950 GPa respectively [23]. Furthermore, the static and dynamic mechanical deflections of MWCNTs performed in a transmission electron microscope, established that the elastic bending modulus was found to decrease from 1 to 0.1 TPa as the diameter of the nanotubes increased from 8 to 40 nm [24].
Therefore, a heretofore unaddressed need exists in the art to address the aforementioned deficiencies and inadequacies.